Battery charger regulation control circuits that force a solar array to operate at a maximum power point of the solar array are generally known as peak power trackers (PPT). PPT circuits have been known for a considerable period of time. However, their general acceptance and widespread use has been hindered by their cost and complexity. One reason for employing a PPT circuit is that the operating characteristics of a conventional photovoltaic solar power array tend to degrade over time. This results in considerably different I-V characteristics at the beginning of the operating life than those exhibited at the end of the operating life.
One significant advantage to using a PPT is the ability to extract the maximum power available from a solar array. Another significant advantage is the ability to extend battery life in the presence of varying battery load or varying solar illumination. This second advantage is made possible by minimizing the depth of the discharge experienced by the battery. Both of these advantages are most beneficial when used in a satellite in a low earth orbit (LEO) that experiences periods of full illumination alternating with eclipse. Thus, despite the added cost and complexity of the PPT, relative to fixed operating point battery charging systems, the use of PPT can be very desirable.
One conventional approach to operating a PPT circuit is known as the disturb and observe approach. In this technique the operating point of the battery charging regulator is offset by some small amount, the power output of the solar array is then determined by multiplying the array output current by the array output voltage, and a search is then made for a maximum in the power output.
A recent example of the disturb and measure approach is described in an article entitled "Design and Analysis of a Microprocessor-Controlled Peak-Power-Tracking System", by P. Huynh and B. H. Cho, Virginia Power Electronics Center, Bradley Electrical Engineering Department, Virginia Polytechnic Institute and State University (August 1992). The authors employ a microprocessor to compute the peak power point. Afterwards, the solar array is forced to operate at a voltage where the output power of the solar array is maximized. The system is said to operate essentially in two different modes: peak-power-tracking (PPT) mode and trickle-charge (TC) mode. As a spacecraft emerges from eclipse, the solar array is regulated at the peak-power voltage (PPT mode) to provide maximum power for the load, and the battery sources or sinks the additional power, depending on the load demand. When the battery is fully charged and the solar array output power exceeds the load power, the PPT system switches to the TC mode. When operating in this mode a small solar array current is used to charge the battery, to compensate for the battery leakage current, while adequate power from the solar array is provided for the load.
Other PPT techniques do not employ a microprocessor, but instead utilize complex analog signal processing which, while eliminating the requirement to provide a programmed microprocessor device, result in considerable complexity and require a significant amount of circuit board area to implement. As is well known, in any earth satellite application the conservation of weight and volume is an important goal.
An object of this invention is to provide a power supply system which provides the benefits of the prior art PPTs, while at the same time doing so at little or no additional cost and complexity, and with a minimized increase in weight and volume.